FOS\UserBundle\Model\UserManager::findUserByUsername PHP Method

    public function findUserByUsername($username)
    {
        return $this->findUserBy(array('usernameCanonical' => $this->usernameCanonicalizer->canonicalize($username)));
    }

 private function testIfUsernameAlreadyInUse()
 {
     $email = $this->userManager->findUserByUsername($this->submitDatas['email']);
     if ($email) {
         throw new \Exception(json_encode(array("email" => "Cet email est déjà utilisé.")), 403);
     }
 }